1. Technical Field
The present invention generally relates to a marine diesel cylinder lubricating oil composition, in particular, for lubricating a marine two-stroke crosshead diesel cylinder engine.
2. Description of the Related Art
In the not so distant past, rapidly escalating energy costs, particularly those incurred in distilling crude oil and liquid petroleum, became burdensome to the users of transportation fuels, such as owners and operators of seagoing ships. In response, those users have steered their operations away from steam turbine propulsion units in favor of large marine diesel engines that are more fuel efficient. Diesel engines may generally be classified as low-speed, medium-speed, or high-speed engines, with the low-speed variety being used for the largest, deep shaft marine vessels and certain other industrial applications.
Low-speed diesel engines are unique in size and method of operation. The engines themselves are massive, the larger units may approach 200 tons in weight and an upward of 10 feet in length and 45 feet in height. The output of these engines can reach as high as 100,000 brake horsepower with engine revolutions of 60 to about 200 revolutions per minute. They are typically of crosshead design and operate on the two-stroke cycle. These engines typically operate on residual fuels, but some may also operate on distillate fuels that contain little or no residue.
Medium-speed engines, on the other hand, typically operate in the range of about 250 to about 1100 rpm and may operate on either the four-stroke or the two-stroke cycle. These engines can be of trunk piston design or occasionally of crosshead design. They typically operate on residual fuels, just like the low-speed diesel engines, but some may also operate on distillate fuels that contain little or no residue. In addition, these engines can also be used for propulsion, ancillary applications or both on deep-sea vessels.
Low- and medium-speed diesel engines are also extensively used in power plant operations. A low- or medium-speed diesel engine that operates on the two-stroke cycle is typically a direct-coupled and direct-reversing engine of crosshead construction, with a diaphragm and one or more stuffing boxes separating the power cylinders from the crankcase to prevent combustion products from entering the crankcase and mixing with the crankcase oil. The notable complete separation of the crankcase from the combustion zone has led persons skilled in the art to lubricate the combustion chamber and the crankcase with different lubricating oils.
In large diesel engines of the crosshead type used in marine and heavy stationary applications, the cylinders are lubricated separately from the other engine components. The cylinders are lubricated on a total loss basis with the cylinder oil being injected separately to quills on each cylinder by means of lubricators positioned around the cylinder liner. Oil is distributed to the lubricators by means of pumps, which are, in modern engine designs, actuated to apply the oil directly onto the rings to reduce wastage of the oil.
One problem associated with these engines is that their manufacturers commonly design them to use a variety of diesel fuels, ranging from good quality high distillate fuel with low sulfur and low asphaltene content to poorer quality intermediate or heavy fuel such as marine residual fuel with generally high sulfur and higher asphaltene content.
The high stresses encountered in these engines and the use of marine residual fuels creates the need for lubricants with high detergency, neutralizing capability and better stability against oxidation based viscosity increase even though the oils are exposed to thermal and other stresses only for short periods of time. Residual fuels commonly used in these diesel engines typically contain significant quantities of sulfur, which, in the combustion process, combine with water to form sulfuric acid, the presence of which leads to corrosive wear. In particular, in two-stroke engines for ships, areas around the cylinder liners and piston rings can be corroded and worn by the acid. Therefore, it is important for diesel engine lubricating oils to have the ability to resist such corrosion and wear.
Accordingly, one of the primary functions of a marine diesel cylinder lubricant is to neutralize sulfur-based acidic components of high-sulfur fuel oil combusted in low-speed 2-stroke crosshead diesel engines. This neutralization is accomplished by the inclusion in the marine diesel cylinder lubricant of basic species such as metallic detergents. Unfortunately the basicity of the marine diesel cylinder lubricant can be diminished by oxidation of the marine diesel cylinder lubricant (caused by the thermal and oxidative stress the lubricant undergoes in the engine), thus decreasing the lubricant's neutralization ability. Oxidation stability is therefore one of the key performance aspects of a marine cylinder lubricant. Oxidation can be accelerated if the marine diesel cylinder lubricant contains oxidation catalysts such as wear metals that are generally known to be present in the lubricant during engine operation.
Typically, marine cylinder lubricants for use in marine diesel engines have a viscosity in the range of 9.3 to 26.1 centistokes (cSt) at 100° C. In order to formulate such a lubricant, a brightstock can be combined with a low viscosity oil, e.g., an oil having a viscosity from 4 to 6 cSt at 100° C. However, supplies of brightstock are dwindling and therefore brightstock cannot be relied upon to increase the viscosity of marine cylinder lubricants to the range of 16.5 to 25 cSt at 100° C. that manufacturers recommend. In addition, Hart's Lubricant World, September 1997, pp. 27-28, (referenced in EP 1967571) discloses that “Due to low-operating speeds and high loads in marine engines, high viscosity oils (SAE 40, 50, and 60) typically are required. Because hydrocracking results in a viscosity loss of the base stocks, marine oils cannot generally be formulated solely with hydrocracked base stocks, but require the use of significant amounts of brightstock. However, the use of brightstock is not desirable because of the presence of oxidatively unstable aromatics.”
One solution to this problem is to use thickeners such as polyisobutylene or viscosity index improver compounds such as olefin copolymers to thicken the marine cylinder lubricants. However, these materials add to the cost of the marine cylinder lubricants. Another solution is to use lower viscosity marine cylinder lubricants; but the wear performance of low viscosity MCLs has not been well investigated.
Another important performance aspect of a marine cylinder lubricant is foaming performance. Foam forms when a large amount of gas is entrained in a liquid. While foaming is desirable in certain applications, such as floatation, washing and cleaning, it can be undesirable in lubricant-related applications where foaming can be an impediment because it leads to ineffective lubrication. The viscosity and surface tension of a lubricant contribute to the stability of the foam. Low-viscosity oils produce foams with large bubbles, which tend to break quickly and be minimally problematic. However, high-viscosity oils, such as those used as marine cylinder lubricants, generate stable foams that contain fine bubbles and are difficult to break. For marine cylinder lubricants, foaming may lead to disturbances in the lubricant film that keeps the piston ring and cylinder liner surfaces separated. Over time, foaming may also accelerate oxidative degradation of the lubricant and in addition may have effects on the transporting and pumping ability of the oil. Therefore, for marine cylinder lubricants, a foaming specification is often in place for the manufactured product.
U.S. Pat. No. 6,103,672 (“the '672 patent”) discloses a polybutene free marine cylinder lubricant containing a major amount of oil of lubricating viscosity and, provided by admixing therewith, minor amounts of a) at least one of a borated dispersant or oil-soluble or oil-dispersible boron compound; and b) one or more overbased metal compounds. The '672 patent further discloses that the polybutene free marine cylinder lubricant demonstrated improved viscometric properties.
U.S. Pat. No. 4,948,522 (“the '522 patent”) discloses a marine diesel cylinder lubricant containing a borated ashless dispersant, an overbased metal compound and a polybutene having a weight average molecular weight of greater than 100,000. The '522 patent further discloses that the marine diesel cylinder lubricant provides which exhibit increased oxidation, wear and deposits.
U.S. Patent Application Publication 2005/0153847 discloses a marine diesel cylinder lubricant composition having a total base number of at least 30 and comprising: (a) at least 40 mass percent of an oil of lubricating viscosity, (b) a detergent prepared from at least two surfactants, (c) a boron-containing dispersant, and (d) a zinc-containing antiwear additive.
WO 2011051261 discloses Teaches lubricating compositions used in internal combustion engines operated under sustained high load conditions, such as marine diesel engines and power applications, comprising base oil and a combination of a sulphonate and phenate detergent. The examples disclosed in WO 2011051261 employ a high molecular weight polyisobutene succinimide at a concentration of 1.5 wt. % of the composition.
A need still remains, therefore, for an improved marine diesel cylinder lubricating oil composition having oxidative stability as well as foam control, which further allows for a reduction in the amount of brightstock that is used in the lubricating oil composition.